1. Field of the Invention
The present invention relates to a driver circuit for driving a row of light emitting elements (e.g., LEDs) built in an electrophotographic printer, a row of heat generating elements built in a thermal printer, or a row of display elements built in a display apparatus, so that the elements are energized selectively and cyclically.
2. Description of the Related Art
For example, Japanese Patent Application Laid-Open No. H09-109459 discloses an electrophotographic printer incorporating an exposing unit in which a large number of light emitting elements are arranged. The light emitting elements may include LEDs, organic electroluminescence (EL) devices, and light emitting thyristors.
For an exposing unit based on LEDs, a single driver circuit a corresponding LED or selectively drives a plurality of LEDs, so that an LED emits light if current flows from anode to cathode of the LED. The light output of an LED depends on the amount of current flowing through the LED. Controlling the amount of current enables adjustment of the light output—hence exposure energy.
LEDs are commonly formed of compound semiconductors. Crystal defects of the compound semiconductor cause variation of output light of the LEDs, which in turn causes variation in print density of individual dots formed by the LEDs. One known method of correcting the variations in print density of individual dots is as follows: Dot compensation data for correcting the variations in the light output of LEDs is previously stored in a memory, and the individual LEDs are driven in accordance with the dot compensation data, thereby minimizing the variations in print density.
The memory is configured to have two bit lines just as the memory cells in a random access memory (SRAM). Data having opposite logic levels are applied to the two bit lines, thereby writing data into each cell. The driver ICs drive a plurality of groups of LEDs so that each group is driven at different timings from others while at the same time dynamically reading the data from the memory.
However, driver circuits that employ the aforementioned existing memory may suffer from the following drawbacks.
The bit lines used for writing data into the memory cells require switching elements for enabling writing of data at different timings. This increases the number of required elements which in turn increases circuit complexity, being an obstacle to cost reduction of the memory. One known way of solving this drawback is to employ a single bit line instead of two bit lines, but is insufficient in that the memory does not properly operate when the supply voltage decreases.